The primary function of blood flow through the lungs is gas exchange. Therefore, it would be advantageous if this function was performed with the optimal use of energy costs. Under normal conditions the hemodynamic properties of the pulmonary circulation and the heart appear to be matched to maximize right ventricular hydraulic power output. This matching depends not only on input resistance, but also on characteristic impedance and pulmonary arterial compliance. The purpose of this study is to determine the changes of hemodynamic properties during acute circulatory disturbances and the mechanisms involved. It is planned to estimate the characteristic impedance and compliance of the main pulmonary artery from measurements of pressure and diameter in both awake and anesthetized dogs. Flow will be calculated from the measurement of blood velocity and vascular dimensions and used to estimate pulmonary arterial input impedance. In awake preparations, it is proposed to determine the uniformity of the mechanical properties of the main pulmonary artery and the role of the autonomic nervous system in controlling the mechanical properties of the main pulmonary artery under resting conditions. In anesthetized preparations, it is proposed to measure pressure-diameter relations of the main pulmonary artery in response to acute circulatory disturbances: (1) alterations of cardiac output, (2) pulmonary vascular obstruction and (3) increases of left atrial pressure. The results of these experiments will be used to determine if the pulmonary hemodynamic variables change in a manner that could be explained by passive mechanical effects. The measurements of vascular dimensions and pressure will enable the relative contributions of geometry and elasticity on vascular compliance to be determined. The role of active mechanisms mediated through the autonomic nervous system will be investigated by the use of blocking agents.